[GeoffR]

I prefer epubs, I think the advantages of the epub reader (better font support for kerning, working hyphenation, better justification, more even word spacing, closer line spacing options) are all things that directly affect reading, while the advantages of kepubs (image zoom, reading stats, better annotation and popup footnotes) are mainly things that are extra to the core function of reading.

Some observations:

the kepub reader does support kerning with some fonts, e.g. Amasis and Gill Sans. I haven't done extensive testing, but my guess is that the kepub reader only supports kerning with ttf fonts. The epub reader is certainly better, it seems to support kerning with both ttf and otf fonts.

I haven't noticed any difference in the typegenius features (font weight/sharpness) between epub and kepub, epub seems to work just as well with otf fonts. It depends more on the particular firmware version and particular fonts. [Edit: I just noticed: with firmware 3.13.1 typegenius doesn't work with opf fonts in the epub reader :-(]

As far as the speed of page turning, it is not a big difference unless the book has excessively large html files (usually multiple chapters in one html file), and then the epub reader slows down noticeably. However those sorts of books cause problems for the kepub reader too: the reading stats and some page numbering options don't work properly when the kepub book has multiple chapters in one file.

The kepub reader has problems with uneven word spacing and justification when there are certain non-ascii characters on the line: curley quotes, elipses, m-dashes, etc.